The present invention pertains generally to integrated circuit fabrication and specifically to integrated circuits containing optical or imaging devices.
Dark pixels are used in light sensing and imaging devices to compensate for components that are generated by causes other than light falling on a light sensor. These signal components may be referred to as dark current. To compensate for the dark current, one or more light sensors (also known as pixels) are prevented from receiving any light. These may be referred to as dark pixels. To compensate for the dark current, the outputs from the dark pixels may be subtracted from the outputs light receiving pixels to remove a component of the dark current from the outputs of the light receiving pixels.
To form a pixel, a photoelectric device is fabricated with a transparent layer. This transparent layer typically allows light to pass through to a light sensing region while still being conductive. By still being conductive, the transparent layer may be may be patterned and used as an electrode or other conductor in the formation of the light sensing or imaging device. This transparent layer may be selected from a group that includes indium tin oxide (ITO), tin oxide, zinc oxide, cadmium oxide, cadmium tin oxide, titanium oxide, and gold thin film.
To form a dark pixel, the light-receiving region of a pixel is covered with a shielding layer that prevents that pixel from receiving light. Prior art devices have constructed this light shielding layer from Aluminum (Al) or an Aluminum alloy such as Alxe2x80x94Si, Alxe2x80x94Ti, Alxe2x80x94Cu, Alxe2x80x94Sixe2x80x94Ti and Alxe2x80x94Sixe2x80x94Cu, or others.
Unfortunately, when it is desirable to place the light-shielding layer over the transparent layer, patterning these light-shielding materials is difficult. Extra processing steps and photomasks may be required to avoid damaging the transparent layer (e.g., ITO) when the light shielding layer is etched using typical etchants for these materials. Accordingly, there is a need in the art for a light shielding layer that may be easily etched without etching the transparent layer.
A preferred embodiment of the invention provides an optical barrier or light-shielding layer. This layer can be applied using known integrated circuit (IC) processing steps, but IC processing techniques developed in the future may also be suitable for applying this light-shielding layer. This layer can also be placed on top of a transparent layer, which preferably is indium tin oxide (ITO), and etched with an etchant that does not react adversely with the ITO. This allows the transparent layer to be an etch stop layer, which simplifies the number of processing steps required to construct a light-shielding layer on top of the transparent layer.
An optical barrier according to the invention preferably is comprised of tungsten (W) or titanium-tungsten (TiW). A layer of the optical barrier material is deposited over the transparent layer, which preferably is ITO. In accordance with one embodiment, the optical barrier material is patterned using known photolithography photoresist, processing steps, and hydrogen peroxide as an etchant. The patterned optical barrier may then act as a light-shielding layer over a light-sensing device to form a dark pixel. In accordance with another embodiment, the optical barrier is patterned using reactive ion etching (RIE), which is a dry etching process. Preferably, the dry etchant is a fluorine-based etchant. Using RIE to etch the optical barrier material layer enables fine line pattern definition of structures to be obtained in the W or TiW without any risk of damaging the transparent layer (e.g., ITO).
Other features and advantages of the present invention will become apparent from the following description, drawings and claims.